The benefits associated with composite structures for use in oilfield equipment, particularly that for wellhead equipment, are well known and documented. Effluent-wetted surfaces of petroleum or petrochemical processing equipment are clad to improve corrosion, heat or wear resistance; to withstand errosion, high pressure or applied stresses; to rebuild worn or improperly sized parts; or to retro-fit old machinery for modern performance standards. Typical clad coatings for oilfield applications include austenitic stainless steels, Fe-base nickel alloys or nickel-base compositions.
There exists a strong economic incentive for cladding a given part rather than making it entirely from corrosion-resistant-alloy (CRA) material for two important reasons; (1) In this specialized industry, massive section-sizes are used and (2) Significantly different material costs do exist for a substrate of low-alloy carbon-steel and a cladding of CRA material. Generally, when a surface coating exceeds about 3% of the total mass of the base, or substrate material, the resultant product shape is called a composite structure and the coating is considered as a cladding.
Typical clad coatings for wellhead equipment are fabricated by either welding or hot-isostatic-pressing (HIP) techniques. Experience with these conventional technologies has shown that a number of problems still exist, including those associated with interfacial delaminations, low bond strengths, significant porosity levels, escessive defects or discontinuities, capital or labor intensive costs, dilution, precipitation hardening and residual stresses.
Diffusion couples resulting from the HIP process can cause extreme segragation and undesired concentration gradients across the interfaces between the can used for containing the loose powder prior to densification and the consolidated clad deposit. A similar problem of undesired diffusion zones can also develop at the clad interface with respect to the underlying substrate material. Weld-deposited clad exhibits less segregation and fewer concentration gradients than HIP-clad structures but welding introduces another disadvantage in the form of a quenched-and-tempered martensitic phase structure at the interface of clad and substrate material.